Thermosensitive recording media are ordinarily prepared by mixing together a leuco dye and a color developing agent, such as a phenolic compound and the like, after grinding them into fine particles, preparing a coating solution by adding a binder, a filler, a sensitizer, a slipping agent and other aids to the mixture and applying the coating solution onto a substrate such as paper, synthetic paper, film, plastic and the like. Thermosensitive recording medium develops color through an instantaneous chemical reaction when heated by a thermal head, hot stamp, hot pen, laser light or the like to yield a recorded image. Such thermosensitive recording media are used extensively in recording media such as facsimile devices, computer terminal printers, automatic ticket dispensers, recorders for meters, receipts at super markets and convenience stores and the like.
In recent years, the medium is becoming smaller in such an application as handy terminal application, then the medium is required to have a more superior color developing property. For example, the medium is required to have a better color developing sensitivity and image quality, especially bar code readability in such an application as labels and tickets. In addition, the medium is required to be superior in preserving property, such as water resistance, plasticizer resistance and the like, which have not been problematic for a conventional thermosensitive recording media. For example, since the medium is often used in outdoor label application or in food application, the medium is required to have a superior readability of the image which is easily deteriorated by water or moisture of rain drops. Furthermore, since tickets are often accommodated in wallet, the medium for tickets is required to have a superior readability of the image which is easily deteriorated by the plasticizers that is often contained in the synthetic leather used for wallets.
As a method of improving the storage stability of image area, it is generally known to install a protective layer on the thermosensitive recording layer. However, when a protective layer is formed on the thermal recording layer, the protective layer absorbs the heat energy from a thermal head or the like, thereby the color developing performance, i.e. color developing sensitivity and image quality, becomes poorer. In particular, as the contrast between the printed area and a blank area is lowered due to the reduced color developing sensitivity, for example, when a bar code is printed on the medium, the printed barcode cannot be read properly by a reading machine, i.e. the bar code readability becomes poor.
On the other hand, it has been implemented to improve the storage stability of the image portion by incorporating two types of color developing agents, a specific sulfonic acid compound and a specific diphenyl sulfone compound, in the thermosensitive recording layer (References 1 to 3 etc.).
Furthermore, it is also implemented to improve the water resistance and the like of the thermosensitive recording medium by using a specific binder and a specific crosslinking agent in a thermosensitive recording layer and a protective layer (References 4 to 6 etc.).